Physiological Adjustments of Sand Gazelles (Gazella subgutturosa) to a Boom‐or‐Bust Economy: Standard Fasting Metabolic Rate, Total Evaporative Water Loss, and Changes in the Sizes of Organs during Food and Water Restriction

Abstract

To test the hypothesis that desert ungulates adjust their physiology in response to long-term food and water restriction, we established three groups of sand gazelles (Gazella subgutturosa): one that was provided food and water (n = 6; CTRL) ad lib. for 4 mo, one that received ad lib. food and water for the same period but was deprived of food and water for the last 4.5 d (n = 6; EXPT(1)), and one that was exposed to 4 mo of progressive food and water restriction, an experimental regime designed to mimic conditions in a natural desert setting (n = 6; EXPT(2)). At the end of the 4-mo experiment, we measured standard fasting metabolic rate (SFMR) and total evaporative water loss (TEWL) of all sand gazelles and determined lean dry mass of organs of gazelles in CTRL and EXPT(2). Gazelles in CTRL had a mean SFMR of 2,524 +/- 194 kJ d(-1), whereas gazelles in EXPT(1) and EXPT(2) had SFMRs of 2,101+/- 232 and 1,365 +/- 182 kJ d(-1), respectively, values that differed significantly when we controlled for differences in body mass. Gazelles had TEWLs of 151.1 +/- 18.2, 138.5 +/- 17.53, and 98.4 +/- 27.2 g H(2)O d(-1) in CTRL, EXPT(1), and EXPT(2), respectively. For the latter group, mass-independent TEWL was 27.1% of the value for CTRL. We found that normally hydrated sand gazelles had a low mass-adjusted TEWL compared with other arid-zone ungulates: 13.6 g H(2)O kg(-0.898) d(-1), only 17.1% of allometric predictions, the lowest ever measured in an arid-zone ungulate. After 4 mo of progressive food and water restriction, dry lean mass of liver, heart, and muscle of gazelles in EXPT(2) was significantly less than that of these same organs in CTRL, even when we controlled for body mass decrease. Decreases in the dry lean mass of liver explained 70.4% of the variance of SFMR in food- and water-restricted gazelles. As oxygen demands decreased because of reduced organ sizes, gazelles lost less evaporative water, probably because of a decreased respiratory water loss.

Towards a mechanistic understanding of the responses of large terrestrial mammals to heat and aridity associated with climate change

“…In Arabian sand gazelles, a decrease in liver, heart, and muscle masses contributed significantly to a decrease in standard fasting metabolic rate when they were foodand water-restricted. The mass of the intestine and rumen did not change, however, perhaps reflecting an adaptation of desert mammals enabling them to adjust rapidly in the event that food becomes available after rain [148]. Whether the responses involved in acclimatization to seasonal changes in food availability will be sufficient to buffer mammals against greater food deficits likely with climate change, or unpredictable changes in food resources, is not known.…”

Repeatability of metabolic rate is lower for animals living under field versus laboratory conditions

“…Metabolic rates are flexible and can change in response to food availability (Ostrowski et al, 2006), diet quality (Naya et al, 2007) and temperature (McKechnie, 2008), but how environmental variation is expected to affect the repeatability of metabolic rate measurements is not well understood. On the one hand, spatial and temporal environmental variability may act to reduce the repeatability.…”

Metabolic rates of three gazelle species (Nanger soemmerringii, Gazella gazella, Gazella spekei) adapted to arid habitats

“…Ostrowski et al (2006a) demonstrated that a reduced food and Table 1 Body mass, measured and expected metabolic rates and respiratory quotients of the three gazelle species measured in this study. water intake leads to a reduction in fasting metabolism in G. subgutturosa.…”

“…Our study is no exception in not testing for this effect; instead, we simply followed the common practice of using measurements for 'RMR' that were taken after the animals had stayed in the respiration chamber for several hours (e.g. Haim and Skinner 1991;Ostrowski et al 2006a). It could be hypothesized that results higher than expected values, as in G. spekei and G. gazella in our study, might be caused by the excitement due to the unusual handling and confinement.…”

“…Horst and Langworthy 1971;Dunson 1974), which enables them to produce less and more concentrated urine (Maloiy et al 1979;Beuchat 1990). With a reduced MR, water loss is also reduced by a lower total evaporative water loss, but this can additionally be influenced by other species-specific factors (Ostrowski et al 2006a(Ostrowski et al , 2006b, such as anatomical adaptation of the nasal passages (Kamau et al 1984).…”